Magnetostriction of the light Rare Earth Metals

The length change of a magnet at the application of a magnetic field, i.e. the magnetostriction is one of the fundamental material properties. Technical applications range from generation of ultrasonic waves to high accuracy positioning devices. In order to optimize such materials a fundamental knowledge of the magnetoelastic interactions in solids is required. Current theories are based on a large amount of magnetostriction data, which is available and it is amazing, that some of the most prominent magnetic materials - the rare earth elements have not been completely characterized. We performed a careful study of the literature since the middle of the last century, when high purity rare earth elements became available. The result is, that among the light rare earth the magnetostriction of Sm, Eu, Ce and Tm has not been reported. We believe that - using todays techniques of sample preparation and our highly advanced experimental methods - the measurement of the magnetostriction of single crystals of the rare earth elements Sm, Tm and Eu requires only the proposed short term project costing 73.000 Euro. The motivation for this project is not only the fact that the magnetostriction in these elements is not availble, we also foresee to get new physical insights by such measuremens. Currently different mechanisms are discussed, which may lead to magnetostriction in rare earth elements and compounds. The traditional view that only crystal field effects are responsible for the magnetostriction had to be dropped because of recently performed experiments on Gd compounds. The measurements we propose in this project may lead to improve our current models and find out new sources of magnetoelasticity. In this context special attention deserves the case of Sm metal, where the spin and orbital momentum compensate. For the measurements we propose to use a miniature capacitance dilatometer, which we have developed in house. It has been well tested, applied to a large number of materials and is in use at several high field laboratories all over Europe. The complete magnetostriction tensor shall be measured by applying a magnetic field up to 11 Tesla and measuring the length changes in the different crystallographic directions. In some cases higher magnetic fields are required and the dilatometer will be used in dedicated high field laboratories (Dresden, Grenoble, Tallahassee).

A billion times stronger than the magnetic field of earth, 50.000 times stronger than a big permanent magnet: the US National High Magnetic Field Lab (NHMF) - Tallahassee, Florida - houses the world`s strongest magnet with a field of 45 tesla. This huge field is produced with a hybrid magnet design consisting of superconducting and "normal" copper coils (carrying a current of 40.000 Amps, cooled by water under a high pressure of 50 bar). The high magnetic field is produced only within a bore of 32mm diameter: exactly there the sample material was placed and it was investigated, how these samples change their shape upon application of the large magnetic field. The length of a crystal can change up to 10% in such an experiment; sometimes this is called phase transition, because the length change occurs very abrupt. This "magnetostriction" is based on the fact, that a magnetic field can change the distances between the atoms in a crystal lattice. The magnetic field acts on the electrons, which carry a spin and an orbital momentum. Due to the nonzero charge of an electron both, spin and orbital momentum produce a magnetic moment. In the FWF project lanthanides (rare earth) were investigated, these are elements such as Cerium (Ce), Neodymium (Nd), Samarium (Sm) and Gadolinium (Gd), which are situated at the bottom region of the periodic table, but are not as rare as their name indicates. These elements are very similar to each other, because they have the same amount of electrons in their outer shells (5d, 6s) and only differ in the number of 4f-electrons, which are situated more inside the atom. Due to this circumstance the 4f-electrons do not "feel" the influence of the neighbouring atoms, they are screened by the s- and d- electrons in the outer shells. Furthermore, the f-electron shell exhibits a large magnetic moment, which determines the magnetic properties. Because of these special circumstances it is possible to relate the magnetic properties of rare earth based systems to the magnetic properties of the rare earth atoms. This simplifies the theoretical treatment and led to the so called Standard Model of rare earth magnetism. Within this model the interaction of the rare earth magnetic moment and the crystal lattice is described by a classical electric field. This crystal field model is not always sufficient to describe magnetostriction, such as in the case of Sm or Gd. In these elements magnetic two ion interactions have to be considered. The mechanism is similar to two bar magnets: trying to put them onto each other in a way that the north poles touch leads to a strong repelling force. If it is possible to rotate one of the magnets by an external magnetic field by 180 degrees, it is easy to assemble the magnets. Similar, if a magnetic field rotates the magnetic moment of an atom, this leads to a force, which changes the distance between atoms. The magnitude of the field required for this rotation depends on the size of the magnetic moment. This is very small for Sm (because spin and orbital momentum nearly compensate) and thus required an investigation in the high magnetic field at the NHMFL. A detailed study of the magnetostriction mechanism on a number of materials led to the discovery of a paradox, which attracts great attention also beyond this scientific field. In addition, the measurement technique of magnetostriction on small samples was accepted for patent and software to model magnetic properties of materials was developed. The city of Vienna got interest in the experience collected within this project and ordered a pilot study for the setup of a high magnetic field laboratory in Vienna similar to the NHMFL. This would be of practical importance: magnetostriction is already applied, such as for the generation of ultrasound and for high precision positioning devices. However, the behaviour of matter in extremely high magnetic fields is not well studied and could exhibit effects relevant for technical application